Smartphone-integrated portable liquid-level and RI sensor based on a coupling technique using POF.

In this article, we report a low-cost method to fabricate plastic optical fiber (POF) liquid-level and refractive index (RI) sensors integrated with smartphones by molds and drilling microholes in side-growing POFs (SGPOFs) with high-temperature metal wire. These sensors can be used for real-time liquid-level and RI monitoring, especially under situations requiring portability and illumination. When the liquid level changes, the change of the RI around the microholes leads to a change in the light intensity transmitted in the fiber, enabling us to measure the liquid level and RI. Utilizing one sensor with six holes (0.45 mm radius and 15 mm spacing) in 2.0-mm SGPOF, the results of level and RI measuring are in good agreement with the theory. The liquid-level measurement of the sensor is carried out for water with an RI of 1.333, the level sensitivity is 0.29%/cm ± 0.02%/cm. In the RI measurement, the RI sensitivity of the sensor is −22.8%/RIU ± 0.6%/RIU in the measurement range from 1.333 to 1.475.

Compatibility defects of the fiber-optic liquid level and refractive index sensors based on modal interference

In this work, a plastic optical fiber (POF) based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and liquid level. The sensor probe is fabricated by polishing the POF and drilling several micro-holes with equal pitch along the fiber axial, after depositing a layer of gold film on the polished region, the SPR probe is obtained. The effects of the micro-hole on the sensing performance are investigated and the structural parameters of the micro hole are optimized. Experimental results show that the changes of the liquid RI can be detected by monitoring the wavelength position of the SPR peak, and the liquid level can be measured by monitoring the deepness of the SPR peak. An RI sensitivity of 2024.41 nm/RIU was obtained in the RI range of 1.335-1.40, and a resolution of 5 mm and measurement range of 25 mm for liquid level measurement were achieved for the probe. The proposed sensor is compact in size and has a very small cross sensitivity for the RI and liquid level measurement, which is useful in the biomedical and industrial sensing fields.

In this paper, a dual-parameter liquid level and refractive index (R.I.) sensor is fabricated using three pieces of bare polymer optical fibers (POFs), which can independently and simultaneously sense the liquid level and R.I. The proposed sensor design utilizes the twisted coupling technique, in which two optical fibers are twisted and coupled with macro-bending. The liquid level measurement depends on the coupling loss, where the light is transmitted to emission fiber (EF) and twisted with coupled acceptor fiber (AF). The proposed liquid level sensor can measure depths up to 125 mm with a sensitivity of 8.03 nW/mm. Moreover, R.I. sensing depends on the twisted coupled macro-bending (TCMB) technique, where the EF generates bend loss and the AF couples the loss, where the coupled power varies due to the transformation in R.I. of coupling medium. The R.I. sensor revealing a notable sensitivity of -2663%/RIU and an impressive resolution of 3.754 × 10−4 in the different NaCl-saturated liquids featuring R.I. range extending from 1.333 to 1.361. The experimental findings indicate the sensor exhibits excellent stability and reliability. The sensor’s straightforward, comprehensive, and cost-effective design enables its application in chemical, petroleum, and other industries.

In this work, a simple side-polish plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and liquid level. The effects of side-polish depths on the sensing performance were studied. The experimental results show that the SPR peak wavelength will be changed as the RI changes, and the SPR peak intensity will be changed with the liquid level variation. By monitoring the changes in peak wavelength and intensity, the RI and liquid level can be detected simultaneously. Experimental results show that an RI sensitivity of 2008.58 nm/RIU can be reached at an RI of 1.39. This sensor has the advantages of simple structure and low cost, which has a good prospect in the field of biochemical sensing.

A novel liquid-level sensor was proposed and studied by using a specialty double-cladding fiber (DCF). The sensor operates according to a light wavelength modulation method which results from the variation of the surrounding refractive index (RI). As the surrounding RI changes, the effective refractive index of cladding mode of DCF increases or decreases and the phase-matching condition changes, so does shift the resonance wavelength. In our experiments, the DCF sensor was exposed to different liquids with certified RI, and liquid levels were varied. Experimental results showed that the shift of the transmission spectra linearly depended on the fraction change of DCF immersed into the liquid. The liquid level sensitivities are 451.8 pm/mm and 1030.44 pm/mm for the different liquid with RI of 1.3514 and of 1.4286, respectively. The sensor has the advantages of large linear response range, as well as high sensitivity and simple structure et al.

A liquid level sensor based on a long-period fiber grating is proposed. The principle of this type of sensor is based on the refractive index sensitivity of long-period fiber grating. As the cladding mode's effective refractive index depends on the refractive index of the surrounding material, the resonant wavelengths of the long period fiber grating will shift when the surrounding refractive index changes. By monitoring a given attenuation band's resonant wavelength shifts, one can measure the immersed lengths of long period fiber gratings and then the liquid level. A direct liquid level measurement is demonstrated by using a long-period fiber grating which was fabricated in a photosensitive B-Ge co-doped optical fiber from Fibercore Inc. (Fibercore PS 1250/1500). The long-period fiber grating was immersed in two solutions which have different refractive indexes. A maximum shift of 7.69 nm for 50 mm of solution which has the highest refractive index has been observed.

We propose here a liquid level sensor using a long period fiber grating (LPFG) in which direct liquid level measurement is carried out by utilising an LPFG, 100mm in length and a periodicity of 1mm. The LPFG was exposed to liquids with varying levels and the wavelength shift of a selected loss band of the transmission spectra was monitored using a broadband light source and an optical spectrum analyzer. The mechanism of this LPFG sensor is based on the fact that the effective Refractive Index (RI) of a cladding mode is directly dependant on the RI of the surrounding medium, be it air, or in this case water and petrol. As the surrounding RI changes, so does the phase matching condition of the LPFG. The result for the level change of the liquid with a specific RI is both a shift in wavelength and a change in the attenuation level of the selected loss band. For the selected loss band, continuous wavelength shifts of 9.5 nm and 25 nm for 100 mm of water and petrol level change have been observed respectively, with sub-millimetre accuracy.

A novel liquid-level sensor was proposed and studied by using a specialty double-cladding fiber (DCF). The sensor operates according to a light wavelength modulation method which results from the variation of the surrounding refractive index (RI). As the surrounding RI changes, the effective refractive index of cladding mode of DCF increases or decreases and the phase-matching condition changes, so does shift the resonance wavelength. In our experiments, the DCF sensor was exposed to different liquids with certified RI, and liquid levels were varied. Experimental results showed that the shift of the transmission spectra linearly depended on the fraction change of DCF immersed into the liquid. The liquid level sensitivities are 451.8 pm/mm and 1030.44 pm/mm for the different liquid with RI of 1.3514 and of 1.4286, respectively. The sensor has the advantages of large linear response range, as well as high sensitivity and simple structure et al.

For nearly a decade, liquid level sensors have been on the market as white food or beverage products, manufacturing, medical, residential, farming, automotive, aerospace, defense and liquid leak detection or level measurement devices. The nano material-resistive sensor based on reduced graphene oxide is highly sensitive and highly reactive to level measurement. This rGO liquid level sensor was developed, and using screen printing process, nano material film is printed on Al2O3 substratum. For safety purposes the sensor film was coated with Parylene-C. The rGO-based nano material was synthesized using Modified Hummer process. A linear relationship between the rGO film’s resistances was observed due to the change in liquid level, which implies fast detection and therefore calibration. With its fast and cost-effective manufacturing process, this liquid level sensor has considerable potential in industries where liquid level sensing is mandatory e.g. pharmaceutical also, space, automotive and even for liquid storage, liquid levels using cryogenic fluids of H2O icewater, LCO2, LN2, LH2, LOX and LHe.

In our previous work, a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed (1). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index (2). In this work, a technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 1×10 -4 of measured index change, with a temperature sensitivity For temperature insensitive sensors based on fiber Bragg gratings, several techniques have been proposed to discriminate between Bragg resonance spectral shifts associated with refractive index measurements and those induced by fluctuations in temperature. These techniques are implemented by using: a second Bragg grating in a side-polished fiber Bragg grating refractometer (3-4), higher order modes in an etched-core of a fiber Bragg grating sensor (5-6), and higher order modes in a tilted fiber Bragg grating sensor (7-12). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index. The relative shift of the core mode resonance to cladding mode resonance is used to measure the refractive index of substances under test. The device fabricated produced a relative resonance shift of 1 pm for every 5×10 -4 of measured index change, with a temperature sensitivity ~ 0.5 pm/°C (2). Taking a similar approach, here, we reported another technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide architecture in order to discriminate between changes in temperature and refractive index. In our previous work (1), a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed. The structure of the open-top ridge waveguide is as shown in Fig.1. The center ridge waveguide with the Bragg grating is tested as a refractometer by coupling the light source into the end of the waveguide. The function of the two adjacent waveguides is to act as a barrier and to partially prevent the liquid from flowing away from the waveguide containing the grating. The guided light of the center waveguide couples evanescently into the surrounding liquid through the top and sides of the waveguide. When a Bragg grating is induced in the core of an open-top ridge waveguide with a larger birefringence, TE and TM resonances are observed when the light guided by the core is phase matched by the grating structure. Both TE and TM resonances are sensitive to the liquid refractive index on the top layer of the open-top ridge waveguide. The TE and TM sensitivities to temperature fluctuations however, are more closely matched. These characteristics can be used to decouple fluctuations of the Bragg resonance of the core mode due to temperature from those changes that are due to variation in the refractive index of the analyte liquid. In the experiments presented here, the variation of TE and TM resonances are investigated as a function of temperature and the external refractive index nt. A theoretical model is developed to investigate the performance of some potential waveguide structures. Relationships between the waveguide core size, refractive index distribution, as

Liquid level and temperature sensor based on an asymmetrical fiber Mach–Zehnder interferometer combined with a fiber Bragg grating

A detail study was done on the sensitivities of 1-D photonic crystal (PC) and 2-D PC coupled cavity sensors with changing sensing layer parameters of thickness and refractive index (RI). Though both refractive index and thickness are interrelated they have significant individual affects on device response. In 1-D PC shifts in normal transmission peak due to surface change in thickness and RI and in 2-D PC coupled cavity shifts in transmission dip due to surface changes are observed. Here sensitivity analysis in change in thickness and RI on these devices was done for four cases; case 1: change in thickness from 2nm-10nm on PC sensors, case 2: change in thickness from 75nm-175nm on PC sensors, case 3: change in RI in thin film (6nm) on surface and case 4: change in RI in thick film (100nm) on sensors surface. Sensitivities due to change in thickness (S_t) of 1-D PC and 2-D PC coupled cavity were calculated from the slope of the sensitivity curves and found to be (for RI of 1.4) 1.423nm/nm and 2.285nm/nm for case 1 and 0.455nm/nm and 0.801nm/nm for case 2. Sensitivities due to change in RI (S_r) of 1-D PC and 2-D PC coupled cavity were obtained from the transmission peak and dip shifts due to change in RI from 1(air) to 2. Sr for 1-D PC and 2-D PC coupled cavity were found to be 70nm/RIU and 103nm/RIU for case 3 and 143nm/RIU and 213nm/RIU for case 4. The results are based on FDTD simulations.

This article reports the principle of operation, design aspects, experimentation, and performance of a noncontact fiber optic liquid level sensor. The sensor is based on the phenomenon of reflective concept. The device consists of fiber optic transmitter, fiber optic probe, floating reflector, photodiode detector, and digital multimeter. The fiber optic probe consists of two 60‐cm‐long PMMA (polymethyl methacrylate) fibers of diameter 1 mm, numerical aperture 0.5, core refractive index 1.492, and cladding refractive index 1.402. The fiber optic sensor is a promising alternative to other well‐established methods for the measurement of liquid level due to its simplicity of design, high precision, long‐term stability, linearity, high degree of sensitivity, dynamic range, noncontact sensing, and low cost of the fabrication make it suitable for applications in precise level control in analytical and process chemistry, biochemistry, bioanalytics, and on‐line measurement or inspection of liquid level. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 2114–2118, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25415

Plastic fiber optic sensor for continuous liquid level monitoring